During the interwar period, ‘physico-chemical biology’ was institutionalised on an unprecedented scale. A group of eminent researchers, science managers and philanthropists promoted the view that physical and chemical concepts and methods could and should be adopted in biology. My talk is concerned with the practical implementation of this vision: how did researchers (from the physical and the biological sciences) identify biological problems that were to be approached from a physico-chemical standpoint? And, after all, why did they decide to work on problems at the interface between the physical and life sciences? I will introduce four interwar research programs in which physical or chemical methods and concepts were used to investigate biological phenomena: research on plant growth hormones in Utrecht and Pasadena; Selig Hecht’s work on the physical and chemical basis of vision; Cambridge biochemist Rose Scott-Moncrieff’s study of the biochemical basis of flower colour inheritance; and the activities of Prague’s ‘biological-physical working group’. The talk will focus on the early phases of these research programs and show how these cross-disciplinary studies were planned, implemented, and evaluated. The analysis emphasises the material and technological conditions of the modern life sciences and, at the same time, provides insights into the methodological norms that shaped scientists’ actual research actions. Secondly, it promises to speak to the motivations behind cross-disciplinary research collaborations. I will argue that researchers were willing to cooperate with practitioners from other disciplines, since they recognised their epistemical interdependence.